The first part of the twenty-first century will be not nearly as stable as the second half of the twentieth, because the world will be not nearly as bipolar as it was during the Cold War. The fight between Beijing and Washington over the Pacific will not dominate all of world politics, but it will be the most important of several regional struggles. Yet it will be the organizing focus for the U.S. defense posture abroad. If we are smart, this should lead us back into concert with Europe. No matter how successfully our military adapts to the rise of China, it is clear that our current dominance in the Pacific will not last. The Asia expert Mark Helprin has argued that while we pursue our democratization efforts in the Middle East, increasingly befriending only those states whose internal systems resemble our own, China is poised to reap the substantial benefits of pursuing its interests amorally—what the United States did during the Cold War. The Chinese surely hope, for example, that our chilly attitude toward the brutal Uzbek dictator, Islam Karimov, becomes even chillier; this would open up the possibility of more pipeline and other deals with him, and might persuade him to deny us use of the air base at Karshi-Khanabad. Were Karimov to be toppled in an uprising like the one in Kyrgyzstan, we would immediately have to stabilize the new regime or risk losing sections of the country to Chinese influence.

We also need to realize that in the coming years and decades the moral distance between Europe and China is going to contract considerably, especially if China's authoritarianism becomes increasingly restrained, and the ever expanding European Union becomes a less-than-democratic superstate run in imperious regulatory style by Brussels-based functionaries. Russia, too, is headed in a decidedly undemocratic direction: Russia's president, Vladimir Putin, reacted to our support of democracy in Ukraine by agreeing to "massive" joint air and naval exercises with the Chinese, scheduled for the second half of this year. These unprecedented joint Russian-Chinese exercises will be held on Chinese territory.

Therefore the idea that we will no longer engage in the "cynical" game of power politics is illusory, as is the idea that we will be able to advance a foreign policy based solely on Wilsonian ideals. We will have to continually play various parts of the world off China, just as Richard Nixon played less than morally perfect states off the Soviet Union. This may well lead to a fundamentally new NATO alliance, which could become a global armada that roams the Seven Seas. Indeed, the Dutch, the Norwegians, the Germans, and the Spanish are making significant investments in fast missile-bearing ships and in landing-platform docks for beach assaults, and the British and the French are investing in new aircraft carriers. Since Europe increasingly seeks to avoid conflict and to reduce geopolitics to a series of negotiations and regulatory disputes, an emphasis on sea power would suit it well. Sea power is intrinsically less threatening than land power. It allows for a big operation without a large onshore footprint. Consider the tsunami effort, during which Marines and sailors returned to their carrier and destroyers each night. Armies invade; navies make port visits. Sea power has always been a more useful means of realpolitik than land power. It allows for a substantial military presence in areas geographically remote from states themselves—but without an overtly belligerent effect. Because ships take so long to get somewhere, and are less threatening than troops on the ground, naval forces allow diplomats to ratchet up pressure during a crisis in a responsible—and reversible—way. Take the Cuban Missile Crisis, in 1962. As the British expert H. P. Willmott has written, "The use of naval power by the Americans was the least dangerous option that presented itself, and the slowness with which events unfolded at sea gave time for both sides to conceive and implement a rational response to a highly dangerous situation."

Submarines have been an exception to this rule, but their very ability to operate both literally and figuratively below the surface, completely off the media radar screen, allows a government to be militarily aggressive, particularly in the field of espionage, without offending the sensibilities of its citizenry. Sweden's neutrality is a hard-won luxury built on naval strength that many of its idealistic citizens may be incompletely aware of. Pacifistic Japan, the ultimate trading nation, is increasingly dependent on its burgeoning submarine force. Sea power protects trade, which is regulated by treaties; it's no accident that the father of international law, Hugo Grotius, was a seventeenth-century Dutchman who lived at the height of Dutch naval power worldwide. Because of globalization, the twenty-first century will see unprecedented sea traffic, requiring unprecedented regulation by diplomats and naval officers alike. And as the economic influence of the European Union expands around the globe, Europe may find, like the United States in the nineteenth century and China today, that it has to go to sea to protect its interests.

The ships and other naval equipment being built now by the Europeans are designed to slot into U.S. battle networks. And European nations, which today we conceive of as Atlantic forces, may develop global naval functions; already, for example, Swedish submarine units are helping to train Americans in the Pacific on how to hunt for diesel subs. The sea may be nato's and Europe's best chance for a real military future. And yet the alliance is literally and symbolically weak. For it to regain its political significance, NATO must become a military alliance that no one doubts is willing to fight and kill at a moment's notice. That was its reputation during the Cold War—and it was so well regarded by the Soviets that they never tested it. Expanding NATO eastward has helped stabilize former Warsaw Pact states, of course, but admitting substandard militaries to the alliance's ranks, although politically necessary, has been problematic. The more NATO expands eastward, the more superficial and unwieldy it becomes as a fighting force, and the more questionable becomes its claim that it will fight in defense of any member state. Taking in yet more substandard militaries like Ukraine's and Georgia's too soon is simply not in NATO's interest. We can't just declare an expansion of a defense alliance because of demonstrations somewhere in support of democracy. Rather, we must operate in the way we are now operating in Georgia, where we have sent in the Marines for a year to train the Georgian armed forces. That way, when a country like Georgia does make it into NATO, its membership will have military as well as political meaning. Only by making it an agile force that is ready to land on, say, West African beaches at a few days' or hours' notice can we save NATO.

And we need to save it. NATO is ours to lead—unlike the increasingly powerful European Union, whose own defense force, should it become a reality, would inevitably emerge as a competing regional power, one that might align itself with China in order to balance against us. Let me be even clearer about something that policymakers and experts often don't want to be clear about. nato and an autonomous European defense force cannot both prosper. Only one can—and we should want it to be the former, so that Europe is a military asset for us, not a liability, as we confront China.

The Chinese military challenge is already a reality to officers and sailors of the U.S. Navy. I recently spent four weeks embedded on a guided-missile destroyer, the USS Benfold, roaming around the Pacific from Indonesia to Singapore, the Philippines, Guam, and then Hawaii.

During my visit the Benfold completed a tsunami-relief mission (which consisted of bringing foodstuffs ashore and remapping the coastline) and then recommenced combat drills, run from the ship's combat-information center—a dark and cavernous clutter of computer consoles. Here a tactical action officer led the response to what were often hypothetical feints or attacks from China or North Korea.

Observing the action in the combat-information center, I learned that although naval warfare is conducted with headphones and computer keyboards, the stress level is every bit as acute as in gritty urban combat. A wrong decision can result in a catastrophic missile strike, against which no degree of physical toughness or bravery is a defense.

Sea warfare is cerebral. The threat is over the horizon; nothing can be seen; and everything is reduced to mathematics. The object is deception more than it is aggression—getting the other side to shoot first, so as to gain the political advantage, yet not having to absorb the damage of the attack.

As enthusiastic as the crew members of the Benfold were in helping the victims of the tsunami, once they left Indonesian waters they were just as enthusiastic about honing their surface and subsurface warfare skills. I even picked up a feeling, especially among the senior chief petty officers (the iron grunts of the Navy, who provide the truth unvarnished), that they might be tested in the western Pacific to the same degree that the Marines have been in Iraq. The main threat in the Persian Gulf to date has been asymmetric attacks, like the bombing of the Cole. But the Pacific offers all kinds of threats, from increasingly aggressive terrorist groups in the Islamic archipelagoes of Southeast Asia to cat-and-mouse games with Chinese subs in the waters to the north. Preparing to meet all the possible threats the Pacific has to offer will force the Navy to become more nimble, and will make it better able to deal with unconventional emergencies, such as tsunamis, when they arise.

Welcome to the next few decades. As one senior chief put it to me, referring first to the Persian Gulf and then to the Pacific, "The Navy needs to spend less time in that salty little mud puddle and more time in the pond."

Robert D. Kaplan is an Atlantic correspondent and the author of Imperial Grunts: The American Military on the Ground, forthcoming in September from Random House—the first of several books he is writing about the armed forces.